This invention relates to overhead doors, and more particularly, to impact resistant window assemblies installed in the overhead doors.
Garage or overhead doors typically include a number of hingedly connected panels that are moved from a vertical position to a horizontal overhead position over tracks. Window assemblies are typically positioned on the uppermost panels. They are designed to enhance the aesthetic appeal of the door while permitting daylight to pass through the door.
Because such doors present such a large surface area, usually to cover the single biggest opening on the house or building, the survival of such a door in a hurricane or storm is very important to the survival of the house. Experience has shown in older houses that when garage doors fail in hurricane winds a lot of additional damage follows. Older garage doors and new ones that are not properly constructed are highly susceptible to wind damage, including buckling, twisting off the tracks, total collapse, and failure due to impact from windborne debris. Commonly, garage door windows may be the portion of the door that is most susceptible to wind or storm damage.
When evaluating a door relative to such issues, there are two primary considerations. One is to be sure the door is rated for the correct wind pressure for the design wind speed of the area in which it is located. The other is to select a door that is also debris rated.
Efforts have been made for some years to improve the structural strength of elements of buildings, including the overhead garage doors, particularly in coastal areas and most particularly in the state of Florida after unexpectedly heavy damage was caused by recent hurricanes. Standards were developed for determining the merits of structures for withstanding damage in storms characterized by high winds, such as hurricanes and tornadoes. In such storms, strong winds entrain debris that may strike structures such as overhead garage doors and windows with considerable force.
An impact from fast moving debris can cause a structure such as a window or door to fail. Failure of a window or door potentially weakens the structure as a whole, and at least increases the likelihood of further damage by permitting wind, rain and possibly additional debris to enter the building. Conventional window glass is readily frangible (i.e., not tempered or laminated safety glass). Thus breaking the glass may leave an unobstructed opening.
The Florida Building Code (FBC-2010) requires houses in Miami-Dade or Broward Counties to be pressure and debris impact rated. Other areas of that state that are part of the windborne debris region defined in the Florida Building Code mandate that the garage door has to be pressure rated and the windows on the garage door have to be pressure and debris impact rated.
The specific standards applicable in South Florida and other jurisdictions are different in different counties, but typically divide a building into zones of different elevation. At low elevation up to 10 m of building elevation, relatively large wind-borne debris might be expected, such as trashcans, lawn furniture, tree limbs, fencing and building elements.
An exemplary standard for the “large missile zone,” or low elevation zone, is found in the Florida Building Code TAS 201-94 Impact Test Procedures (2010) as well as Department of Energy (DOE) Standard 1020. This standard requires a door or window to survive test firing of a framing stud or a 2×4 lumber missile endwise at the test specimen, at a missile speed of 15 m/s (50 ft/sec or 35 mph). The criterion for a successful test under the FBC is that the test specimen rejects such a missile without any penetration. After the large missile impact tests, which may fracture the glass as discussed above, the test specimens are subjected to an extended series of many positive and negative wind pressure cycles. Similarly, ASTM Standard Specification E1996-12a and ANSI/DASMA 115 set forth a large missile level D in Table 2 of a 2×4 lumber at 50 feet/second. This is believed to have 350 ft-lb of energy for such an impact.
Door and window structures that can routinely survive such tests are available. Some are characterized by a transparent or translucent pane that is inherently strong enough to survive an impact and is mounted rigidly in a door, wall or other structurally sound panel via a rigid mounting structure that likewise can survive the impact. Other windows are laminates of materials and may have layers of glass and flexible plastic, metal or fibrous mesh reinforcement, etc.
The door light, glazing or window pane may be made very durable in and of itself, and/or the pane can be mounted in a manner intended to absorb impact stress. Even assuming the breakage of a frangible glass portion of the door light or window, the envelope that is defined by the window can be designed to remain intact. However, many such impact restraint overhead doors and associated windows are costly, unsightly, aesthetically unpleasing and difficult to install and/or assemble.
What is needed is an impact-resistant window structure for an overhead door, including the ability to survive impact, but also including unit cost, replacement cost, ease of installation, and attractiveness. The mounting for the window should provide a rigidly durable structural engagement for panes of glass or other glazing materials in the overhead door and also the resilient yet durable impact absorbing and resisting capabilities required by applicable building codes.